Organic/metallic hybrid polymers are supramolecular polymers in which metals are arranged precisely in a polymer main chain structure, and are known to exhibit electrical and optical functions by the electronic interaction between the organic ligand and the metal, or by the electronic interaction between adjacent metals.
For example, organic/metallic hybrid polymers containing a transition metal such as iron (Fe) or ruthenium (Ru) exhibit electrochromic properties in the visible region, and have been studied as display device materials (Non-Patent Document 1). It should be noted that the electrochromic property refers to a property to change its color by applying a charge to the material.
Further, organic/heterometallic hybrid polymers into which europium (Eu) and Fe are precisely introduced alternately using asymmetric organic ligands have been reported to function as display devices of the luminescence switching of Eu corresponding to the electrochromic properties by the oxidation-reduction of Fe (Non-Patent Document 2).
In addition, several organic/metallic hybrid polymers having electrochromic properties have been synthesized (Patent Documents 1 and 2).
Although the research and development of electrochromic materials in the visible light region have advanced as described above, the research and development of electrochromic materials in the ultraviolet region or infrared region have not advanced much. In particular, if the electrochromic materials in the infrared region can be developed, they can be applied to a near infrared light-shielding electrochromic window which can freely shield near infrared light of sunlight entering inside the room, thereby enhancing the cooling effect of the room and saving energy. Further, with respect to near infrared light used in optical communications, development of products in the form of an optical device for a near infrared optical shutter may also be possible.
Accordingly, the inventors of the present invention have newly synthesized an organic/heterometallic hybrid polymer in which hetero metals are closely arranged within the polymer via a π-conjugated organic portion, aimed at electrochromism in the near infrared region using the electronic interaction between the metals. The organic/heterometallic hybrid polymer exhibited an intervalence charge transfer (IVCT) absorption between hetero metals (zinc (Zn)—Ru) in the near infrared region by the application of a voltage. Since the absorbance of the absorption changes in a reversible manner by the ON/OFF of the voltage application, the potential as an optical device material was shown.
However, since the energy of IVCT occurring between hetero metals is relatively large, the absorption wavelength is limited to the near infrared region from 900 nm to 1,500 nm. Therefore, it is difficult to realize electrochromism in the infrared region with a wavelength longer than 1,500 nm.
Further, because the electrochromism itself is a simple ON/OFF of the absorption, switching of a plurality of absorption wavelengths required in the optical device of the near infrared optical shutter is difficult.
Furthermore, with respect to the above linear organic/heterometallic hybrid polymers, when the ON/OFF of the voltage application is repeated, the polymer film is gradually dissolved. In other words, durability of the polymer film to the repetitive electrochromic changes is not sufficient.